Investigation of the possibility of using photoneutron beams for radiation therapy.

The possibility has been investigated of using electrons accelerated by a 50 MeV racetrack microtron for generation of photoneutron beams for radiation therapy. Central axis depth-dose curves have been measured in an A-150 tissue-equivalent phantom. Neutron half-value depths between 4.4 and 5.2 g cm-2 were obtained at an SSD of 100 cm for different converter materials and target geometries. At an absorbed dose ratio of 1:1 for neutrons and photons at the dose maximum, the total absorbed dose rates are estimated to be 0.1 Gy min-1 at 100 micronA electron current and a SSD of 100 cm. At a depth of 5 cm the neutron to photon absorbed dose ratio is typically 1:2 and the OER is expected to be about 1.8. Some dose distributional and radiobiological advantages of a physically mixed beam of neutrons and photons for external beam radiation therapy are discussed.